1. Field of the Invention
The present invention relates to a fuel cell stack constructed by alternately stacking separators and fuel cell units each comprising a solid polymer ion exchange membrane interposed by an anode electrode and a cathode electrode.
2. Description of the Related Art
The fuel cell of the solid polymer ion exchange membrane type is constructed as a fuel cell stack comprising a plurality of fuel cell units and a plurality of separators which are alternately stacked, each of the fuel cell units comprising an electrolyte composed of a polymer ion exchange membrane and further comprising a catalyst electrode and a porous carbon electrode which are arranged on both sides of the electrolyte respectively.
Such a fuel cell stack is operated as follows. That is, hydrogen is supplied to an anode electrode, and it is converted into hydrogen ion on the catalyst electrode. The hydrogen ion is moved toward a cathode electrode via the electrolyte which is appropriately humidified or via the electrolyte which is immersed with strong acid. Electron is generated during this process, and it is extracted to an external circuit so that the electron is utilized as DC electric energy. Oxygen-containing gas, for example, oxygen gas or air is supplied to the cathode electrode. Accordingly, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode to produce water.
In order to ensure an effective power-generating function of such a fuel cell stack, it is necessary that the electrode power-generating section (power-generating surface) of the fuel cell unit is maintained to be within a predetermined temperature range. For this purpose, it is generally conceived that the heat is absorbed from the electrode power-generating section by using a flow passage provided in the separator for allowing a cooling medium to flow therethrough. Specifically, as shown in FIG. 6, a cooling medium inlet 3 and a cooling medium outlet 4, which are disposed at outer circumferential edge portions, are provided in a flat surface 2 of a separator 1 which is opposed to an anode electrode or a cathode electrode. The cooling medium inlet 3 communicates with the cooling medium outlet 4 via a flow passage 5 formed in the flat surface 2. As shown in FIG. 6, the flow passage 5 is constructed such tat the cooling medium flows in a meandering manner from the lower part to the upper part.
However, in the case of the separator 1 described above, the cooling water flows along the flow passage 5 while absorbing the heat from the power-generating surface. Therefore, the temperature of the cooling water is raised on the downstream side (at upper portions in FIG. 6) of the flow passage 5. For this reason, the following problem is pointed out. That is, a temperature gradient appears over the power-generating surface of the cell in a certain direction as shown in FIG. 6, which causes unevenness of the power-generating performance over the power-generating surface, resulting in deterioration of the performance of the entire fuel cell stack.
In view of the above, for example, as disclosed in Japanese Laid-Open Patent Publication No. 8-45520, a solid polymer type fuel cell is known, in which a cooling water supply port and a cooling water discharge port are arranged at a central portion or a circumferential edge portion of a polymer ion exchange membrane as viewed in a plan view, and the cooling water supply port communicates with the cooling water discharge port via one passage having a spiral configuration.
In the case of the conventional technique described above, when the cooling water supply port and the cooling water discharge port are arranged at the central portion of the polymer ion exchange membrane, then the cooling water flows in a spiral manner from the central portion to the outer circumferential portion of the polymer ion exchange membrane, and then the cooling water is returned from the outer circumferential portion toward the central portion in the spiral manner. On the other hand, when the cooling water supply port and the cooling water discharge port are arranged at the circumferential edge portion of the polymer ion exchange membrane, then the cooling water flows from the circumferential edge portion to the central portion of the polymer ion exchange membrane in the spiral manner, and then the cooling water is returned to the circumferential edge portion.
However, when the fuel cell stack is installed in the atmospheric air at the room temperature, the heat is released to the outside from the outer circumferential portion of each of the fuel cell units. Therefore, the temperature tends to be lowered at the outer circumferential portion as compared with the central portion. For this reason, the following problem is pointed out. That is, when both of the cooling water supply port and the cooling water discharge port are arranged either at the central portion or at the circumferential edge portion of the polymer ion exchange membrane, the temperature distribution is uneven over the entire power-generating surface of the polymer ion exchange membrane.